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Full text of "Handbook Of Chemical Engineering - I"

REFRACTORIES
513
since the amorphous forms of carbon are more soluble in the molten metal than crystallized graphite, and hence cause too much cementation. Their more rapid rate of oxidation is likewise detrimental for this purpose.
Some of the Birmingham crucibles are said to be made up of 3 parts (by weight) of graphite, 2 of hard coke, 2 of Stourbridge clay, and 1 of ground sherds from old crucibles. Crucibles used in a German foundry have been made from a mixture consisting of 5 parts of ground coke, 4 of graphite, and 8 of Stourbridge clay and grog. If the necessity arose, it would be, of course, possible to use crucibles in which all the carbon is introduced in the form of retort carbon, coke, or electric-furnace graphite, even though the results were not as satisfactory as those obtained with the natural graphite.
Crucible Mixtures.—The main requirements of a suitable crucible mixture are that it must possess satisfactory working qualities, good strength in the dried state, ability to withstand sudden temperature changes, good thermal conductivity, freedom from pinholes, sufficient mechanical strength to stand up under rough treatment with tongs, and slow oxidation of the graphite.
The clays most desirable for this purpose are those of the dense-burning type, which soften slightly at the heat of the brass furnace. The open-burning clays are not suitable for this purpose, though they may be used for the purpose of stiffening clays deficient in refractoriness. The clays to be considered here are those of the Klingenberg type; some of the English ball clays, such as that marketed as "Dorset;" the ball clays from Mayfield, Ky.} and the Paris, Tenn., district; the bonding clay mixtures marketed at St. Louis, Mo.; the plastic bond clays from northern Mississippi, and Anna, 111. In every case those clays are to be preferred which vitrify close to 1,150°C.
The composition of graphite crucibles varies considerably with reference to whether they are to be used for brass or for steel melting. Some of the mixtures which have come to the knowledge of the writer are as follows, the first four being principally intended for melting brass and the last two for steel:
TABLE 8
			Per	cent		
	a	b	c	d	e	/
Graphite ...............	48	57.5	55	50	58	55
Bond clay	32	25 5	35	40	35	30
Kaolin, plastic        ...	6	10 4	5			7
Sand                      ......				5	7	8
Crushed pot shell .........	14		5	5		
Potter's flint    ..........		6.6				
						
The addition of sand can be avoided by the use of siliceous clay, like that from Arkansas (Lester), which makes possible a reduction of the clay content, if this is desirable, and yet the maintenance of good working quality.
Manufacture and Use of Graphite Crucibles.—The preparation of the graphite consists usually in grinding it by means of a burrstone mill or pan or disintegrator and sizing it by means of sieves. A smaller amount of the dust is used, which is frequently utilized for nozzles, stoppers, and other articles. The sized mineral is
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